Investigation of C1q-binding circulating immune complex correlates in osteoarthritis of the knee: a single-center retrospective study

Osteoarthritis of the knee is a common musculoskeletal disease with an increasing incidence and prevalence. Over the last two decades, an increasing weight of inflammatory factors have been emphasized. The activation of the complement system is well known in rheumatological diseases of inflammatory or auto-immune origin; however, its participation in osteoarthritis is less characterized. The goal of our study was to detect the C1q binding circulating immune complexes and to describe their potential clinical and laboratory correlations. 61 patients with knee osteoarthritis and a control group of 17 non-osteoarthritic patients from 2023–2024 were included. Demographic data, as well as patient history, comorbidities and the WOMAC (Western Ontario and McMaster Universities Arthritis Index) disease activity index were recorded. Patients’ complete blood counts, titres of complement factors C3, C4 and levels of circulating immune complexes C1q, the neutrophil-to-lymphocyte ratio (Neu/Ly) and Systemic Inflammatory Response Index (SIRI) were determined. The study group consisted of 86% female patients with a mean age of 66.1 ± 1.1 years. The WOMAC score had a mean value of 24.9 ±1.3. The neutrophil-to-lymphocyte ratio was 2.05 ±0.17, C3,C4 were within the reference range. The C1q CIC levels in the osteoarthritis group studied were slightly above (19.6±2.7 μgEq/mL) the reference cut-off value (<16 μgEq/mL) indicated by the manufacturer. The C1q CIC level of the patient group was significantly higher (p=0.0012) compared to the control group (6.06±0.85 μgEq/mL). C1q CIC did not show a significant correlation with the WOMAC index or with Neu/Ly, SIRI, C3, C4 titers. In contrast, a significant correlation was found between WOMAC and patient age (r=0.28, p=0.026). C1q binding circulating immunocomplex levels were found elevated in patients with osteoarthritis of the knee joint. We found no correlation between C1q CIC and WOMAC score, blood-derived indices and C3 and C4 complement titers.